The present invention relates to a method and a device for spectral level balancing in wavelength division multiplexing (WDM) multi-channel systems.
Spectral level balancing is described, for example, in H. HULTZSCH: Optische Telekommunikationssysteme [Optical Telecommunications Systems]; Gelsenkirchen 1996, p. 344 et seq.
WDM multi-channel systems employ a larger number of equally spaced spectral channels for optical transmission in the glass-fiber network. The transmission capacity is increased through parallel transmission, without the transmission frequencies having to be raised, i.e., a 2.5 Gbit system can be used with 20 channels for the transmission of 50 Gbit without changing the electronic components.
In general, WDM systems operate in the third optical window of telecommunications at 1550 nm, where there are suitable optical amplifiers capable of amplifying the entire multi-channel system, channel by channel. Although the amplifier basically does not distinguish between individual channels, the amplification curve V (xcex) is dependent on the spectral position of the individual channel. If a number N of amplifiers is required for greater distances, the amplification VN (xcex) rises exponentially, which, even given a modest wavelength dependence V(xcex)xc2x7I(xcex), may lead to considerable dependences of the intensity VN(xcex)xc2x7Io(xcex) at the end of the transmission link. If these are no longer tolerable, suitable filters must be used to ensure a uniform spectral dependence of VN(xcex)Io(xcex)xc2x7Io(xcex) is the intensity of the channel of wavelength xcex at the input of the amplifier chain.
For spectral balancing, colored glass filters or interference filters of transmittance D(xcex) may be employed to level the channel intensities D(xcex)VN(xcex)Io(xcex) at the output. Such filters operate completely analogously to a matching filter for the color temperature of the light source to a photographic film. Their reciprocal characteristic reduces the intensity of the intense spectral channels to that of the weak ones. This works with photographic film, but not in the case of a WDM multi-channel system, because the curves V(xcex) differ greatly from amplifier to amplifier and are also time-dependent. Actually, the intensity IA(xcex) at the output of the WDM transmission system is therefore described by                                           I            A                    ⁢                      xe2x80x83                    ⁢                      (                          λ              ,              t                        )                          =                  (                                    ∏                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                          Vi              ⁢                              xe2x80x83                            ⁢                              (                                  λ                  ,                  t                                )                            ⁢                              xe2x80x83                            ⁢                              I                o                            ⁢                              xe2x80x83                            ⁢                              (                                  λ                  ,                  t                                )                                                                        (        1        )            
Such a characteristic cannot be balanced by one single static filter. Even in the case of low time-dependence, the great dependence of the amplification curve Vi(xcex) on the amplifier i and the dependence of IA on the number of amplifiers N makes the development of a uniform level-balancing filter very difficult. This problem is made worse by other channel-dependent dispersion and damping effects, such as polarization mode dispersion.
Thus, for example, for balancing the spectral filters for WDM transmissions, in German Patent Application Nos. 432 7 103 A1 and 432 6 522 A1 multi-beam interferences of many parallel waveguides with individual phase control are proposed. These filters must be actively electronically modulated. This also purportedly holds true for filters able to be modulated electro-optically on the basis of non-linear optical materials, the filters being structured as a function of location, as described in German Patent Application No. 31 48 427 A1.
This also purportedly holds true for the filters described in Parker et al, xe2x80x9cDynamic Holographic Spectral Equalization for WDM.xe2x80x9d A device for spectral level balancing in WDM systems is described, the device having the following features: a single-mode fiber, holographically splitting filters as spectrally splitting elements, lenses as imaging elements and a ferroelectric liquid-crystal cell as part of an spatial light modulator (SLM).
Notably, these devices purportedly need external control using a plurality of electrical control signals which must be derived from the different intensities of the channels of the light signal and which can result in considerable expenditure.
The present invention provides a method for spectral level balancing in a wave division multiplexing multi-channel system. The method includes projecting light from a first fiber using at least one spectral splitting and at least one imaging element so as to split the light into a plurality of channels; then locally projecting the light onto the surface of an optical limiter including an optically addressed spatial light modulator so as to weaken at least one first channel of the plurality of channels more than at least one second channel of the plurality of channels using a nonlinear transmission characteristic of the optically addressed spatial light modulator; and then imaging the plurality of channels onto a point and coupling the plurality of channels into a second fiber.